On the interaction between [Ru(NH3)6]3+ and the G-quadruplex forming thrombin binding aptamer sequence.

The interaction between the thrombin binding aptamer (TBA), a G-quadruplex forming DNA sequence, and the electroactive hexaammineruthenium(III) cation has been studied by electrochemical methods and circular dichroism spectroscopy. When TBA is immobilised on a gold surface in a typical aptasensor configuration, the Ru(NH3)6 cation can be bound to the electrode surface through its interaction with the TBA sequence. This interaction is strong enough to enable the ruthenium complex to remain at the surface when the electrode is immersed in an electrolyte free of Ru(NH3)6, meaning that the complex does not diffuse back into the solution. A stoichiometry of 2 Ru(NH3)6 per TBA strand has been determined, indicating that the interaction differs from the conventional, non-specific electrostatic charge compensation, for which a 5 to 1 ratio would be expected between the triply charged cation and the 15 bases sequence. It is shown that this interaction takes place not only at the surface, but also when both TBA and hexaammineruthenium(III) are dissolved in solution. Under such conditions, a similar stoichiometry of 2 Ru(NH3)6 per TBA strand has been evidenced by two independent methods, namely circular dichroism spectroscopy and differential pulse voltammetry.